How to get the mirror image of an empty?
It would seem that the only way to do this is by mirroring, then inverting the rotation manually:

Mirror your empty with CtrlM:


Invert the rotation of the mirrored empty by adding a - (or removing it if the rotation is already negative):



You can automate inverting the rotation with a simple python script:
import bpy

ob = bpy.context.object

ob.rotation_euler.x = ob.rotation_euler.x * -1
ob.rotation_euler.y = ob.rotation_euler.y * -1
ob.rotation_euler.z = ob.rotation_euler.z * -1

To use, first mirror the object with CtrlM, then run in the text editor with the mirrored object selected.